1. Field of the Invention
The present invention relates to an malfunction diagnostic device and an malfunction diagnostic method for NOx sensors, and in particular, relates to a device and a method of malfunction diagnosis for NOx sensors disposed downstream of a NOx storage reduction catalyst.
2. Description of the Related Art
In general, a NOx catalyst that purifies NOx (nitrogen oxides) contained in the exhaust gas is provided in the exhaust gas purification system of the exhaust system of an internal combustion engine such as a diesel engine and a lean-burn gasoline engine. Several types of NOx catalysts are known; however, among these, the NOx storage reduction catalyst (NSR: NOx Storage Reduction) that adsorbs and remove NOx present in the exhaust gas is well known. The NOx storage reduction catalyst adsrobs NOx in the exhaust gas when the air-fuel ratio of the exhaust gas supplied thereto is leaner (when an excess amount of oxygen is present in the atmosphere) than a predetermined value (theoretical air-fuel ratio, in typical), and releases the stored-NOx and reduces it to N2 when the air-fuel ratio of the exhaust gas supplied thereto is richer (when an insufficient amount of oxygen is present in the atmosphere) than the predetermined value. In other words, the NOx storage reduction catalyst has a storage and release function.
Once the NOx storage reduction catalyst stores NOx to a saturation (full) state, the NOx catalyst cannot store NOx any more. For this reason, reducing agents are supplied to the NOx catalyst at appropriate time intervals to place it in the atmosphere where oxygen is insufficient. Stored-NOx is then released from the NOx catalyst to regain a NOx storage capability of the NOx catalyst. This action is called NOx catalyst regeneration.
For example, to determine the timing of start and end of this NOx catalyst regeneration, the NOx sensor that detects the NOx concentration in the exhaust gas is disposed at the downstream side of the NOx catalyst. For example, when the NOx catalyst stores NOx to the full state, NOx leaks into the catalyst downstream side. So, NOx catalyst regeneration may be started when the NOx sensor detects this NOx leakage, for example. When the NOx concentration detected by the NOx sensor is lowered enough during NOx catalyst regeneration, it may be considered that stored-NOx is all released. In this case, NOx catalyst regeneration may be ended.
Here, for engines equipped in automobiles, abnormality detection of catalysts or sensors while mounted in a vehicle (in the on-board state) is required by regulations of various nations in order to prevent automobiles from driving with a bad exhaust gas condition. There exist already relatively many techniques for malfunction detection of catalyst. However, there is no effective technique for malfunction detection of the NOx sensor disposed at the downstream side of the NOx catalyst. Especially now when emission control is getting more severe, requirement expands not simply to the failure detection of a broken wire or the like but also to the accurate detection of rationality of sensor output in relation to deterioration. Therefore, a drastic countermeasure is necessary to satisfy this requirement.
To diagnose malfunctions of for the NOx sensor, for example, a plurality of NOx sensors are disposed and the detected NOx concentrations are compared relatively, or NOx sensors are removed and tested by a stationary type analyzer, may be considered. However, the former method requires high cost, and the latter method cannot perform diagnosis on board.
An malfunction detection device for the NOx sensor that is disposed at the downstream side of the NOx absorbent is disclosed in Japanese Patent Application Publication No. 2003-120399 (JP-A-2003-120399). If NOx concentration in the exhaust gas that reaches the NOx sensor is compulsory changed, and if the change of the NOx sensor output value is deviated from the change of normal sensor operation, malfunction of the NOx sensor is determined.
However, the exhaust gas that reaches the NOx sensor is the exhaust gas that has passed through NOx absorbent. Therefore, the NOx concentration of the exhaust gas is the concentration of NOx in which NOx has been absorbed by the NOx absorbent. In other words, the output value of the NOx sensor reflects the influence of the NOx absorbent that is located in front of the sensor. It causes decreasing accuracy of malfunction diagnosis of the NOx sensor.